1. Field of the Invention
The present invention relates to the hydrolysis of organodichlorosilanes in the presence of an amphoteric surface-active agent.
2. Description of the Prior Art
The hydrolysis of organodichlorosilanes, in particular of dimethyldichlorosilane, to produce cyclic and linear polydimethylsiloxanes, has long been known to this art and is described in numerous patents and publications.
Thus, hydrolytic processes in the liquid aqueous phase with production of gaseous HCl are described, for example, in U.S. Pat. No. 2,483,983 and in French Pat. No. 1,584,040.
Furthermore, it too is known to this art to carry out such hydrolyses in the presence of a cationic (U.S. Pat. No. 3,983,148) or anionic surface-active agent (U.S. Pat. Nos. 4,412,080, 4,423,240 and 4,447,630).
In all instances, the surface-active agent is employed for the purpose of increasing the content of cyclic dimethylpolysiloxanes.
According to U.S. Pat. No. 3,983,148, the cationic surface-active agent is a quaternary ammonium salt, protonated amine or protonated quaternary phosphonium salt.
These surface-active agents do, indeed, increase the content of dimethylcyclopolysiloxanes, but they display at least one of the following two disadvantages:
(1) They are not completely insoluble in the polysiloxanes and they are found in excessively high concentrations in the latter, despite repeated washings;
(2) The resulting aqueous emulsion is generally too stable and it is difficult to separate the aqueous phase from the siloxane phase without conducting an additional treatment, for example heating, which may adversely affect the preservation of the cyclic polysiloxanes produced.
While the anionic surfactants described in the aforementioned three U.S. patents are poorly soluble in the siloxane phase and do not display the disadvantage (1), they do indeed exhibit the disadvantage (2) and, in addition, during hydrolysis they exert detrimental influence on the degree of polymerization of the cyclic siloxanes produced. In point of fact, the trimers and tetramers D.sub.3 and D.sub.4 tend to be converted into their higher homologs D.sub.5 and D.sub.6, as described, furthermore, in U.S. Pat. No. 4,412,081. This may prove disadvantageous, especially during the separation of the aqueous and siloxane phases. This disadvantage may also exist in the case of the cationic surfactants, as reflected in U.S. Pat. No. 4,556,726. Moreover, the cyclic compounds D.sub.x, where x is greater than or equal to 5, are less stable when heated and/or in the presence of an RSO.sup.-.sub.3 anion and have a tendency to transform into linear siloxanes.